Power generation by solar power, wind power, and other natural energy resources, which emits no carbon dioxide, has recently been promoted. However, the amount of power generation by natural energy is subject to natural conditions such as climate and weather. In addition, since it is difficult to adjust the amount of power generation to meet demand, electric-load leveling is needed. Thus leveling generated electric energy through charging and discharging requires a high-energy-density, high-efficiency, and high-capacity storage battery. Molten-salt batteries, which use a molten salt electrolyte to meet such requirements, have been attracting attention.
A molten-salt battery includes an electric generation element composed of, for example, a positive electrode in which a current collector contains an active material of sodium compound particulates, a negative electrode in which a current collector is plated with metal such as tin, and a separator impregnated with a molten salt composed of an alkali metal cation, such as sodium or potassium, and an anion containing fluorine. The separator is arranged between the positive electrode and the negative electrode. In such a molten salt, the free-ion density of the cation is several orders of magnitude higher than that in non-aqueous (oil-based) electrolytes used for lithium ion batteries. It is therefore possible to increase the thickness of the positive-electrode and negative-electrode active materials to achieve a higher capacity.
Sodium-sulfur batteries have undergone many trials to increase the size and therefore the capacity of a cell (see Patent Document 1, for example). Also, Patent Document 2 discloses a technique of stacking plate-type sodium-sulfur cells to provide a space-saving stacked-type molten-salt battery.
On the other hand, lithium ion batteries, which have low electrolytic conductivity, are required to have positive and negative electrodes with large areas to collect a greater current. For this reason, rolled forms are common in which positive and negative electrodes are wound with a separator therebetween and, among such forms, a thin electrode form is known to be effective in which an active material is applied to a current collector composed of a thin metal foil. For example, Patent Document 3 discloses a high-power and high-energy-density non-aqueous secondary battery that uses an aluminum foil with a thickness of 1 to 100 μm.